Even though there are significant advances made in recent years on the technologies of manufacturing and implementing the electromechanical actuators, sensors and micromirror devices as spatial light modulator; there are still technical limitations and difficulties in the manufacturing process. There is a difficulty in a process of releasing the sacrificial layers due to the fact it is difficult to release the sacrificial layers without causing damages to other structures. Specifically, when the etchant for releasing the sacrificial layers is strong acid such as HF, the structure is often damaged, because the etchant can penetrate protective layers.
MEMS devices have drawn considerable interest because of their application as sensors, actuators and display devices. MEMS devices often have a structure as shown in FIG. 1 where an electronic circuit is formed on a substrate and the circuit provides voltage or current to electrodes or senses voltage or current from the electrodes. MEMS structures are often formed over the top or close to the electrodes with a gap between the electrodes and MEMS structure.
The gap is often formed using a technique of sacrificial layer formation followed by a release etching process. This process is exemplified in FIG. 2. FIG. 2A shows that electronic circuit is formed on a substrate. FIG. 2B shows the configuration of the electrodes formed by the processes of metal deposition followed by the processes of metal patterning and etching. Then sacrificial material is deposited over the electrodes. The surface of the sacrificial layer is often polished to form flat surface. Mechanical structures are formed over or in the sacrificial layer. The sacrificial layer is later removed to form a gap between the electrodes and the mechanical structure. This removal process of sacrificial layer is often referred to as a “sacrificial layer release process”.
An example of MEMS structure is shown in FIG. 3, wherein a hinge and a mirror are formed over the top of the electrodes. When voltage is applied to one of the electrodes, the mirror is pulled toward the electrode by an electrostatic force and deflected to a deflection angle because of the hinge that supports the mirror is flexible.
During the sacrificial layer release process, an etching agent is applied to remove only the sacrificial layer without attacking other structures that are supposed to remain intact. However, there are often damages to the structures in addition to the sacrificial layers due to the damages caused by the attack of the etching agent to other structures too. There are several ways to avoid these damages. One of the methods is to use etchant that attacks and removes only the sacrificial layer but does not attack other structures. However, it is often difficult to find such etching agent. Another way is to apply an etch stop layer, which covers the remaining structures with a layer consisting of material which is not attacked by the etchant. When the surface of protective layer is flat, there are many choices of material and deposition methods. However, when the surface is not flat and having topographical shapes, it is very difficult to stop the leak of the protection layer and the etchant penetrates the protective layer through the leak and destroys the structures under the protective payer.
FIG. 1A shows an example of a planar etch stop layer (104) of a micromirror with CMOS transistors (114) in a substrate (111), metal layers (105, 108, 109) for electrical connections for circuitry, electrodes (102) providing voltages to an MEMS mirror (101) having a vertical hinge (103). There is Inter-Layer Dielectric (ILD) shown at 106, 107 and 108 as insulating material. The ILD is often made of SiO2 and receives attack by etchant such as HF. An example of a planar etch stop layer is shown at 104. FIG. 1 illustrates an example of a MEMS structure such as micromirror with electrical wiring (105, 108, 109), a transistor (114), electrodes (102), a vertical hinge (103) and a mirror element (101). The fabrication process requires the use of sacrificial layers, which are filled among final structures including a mirror, a hinge, electrodes and a back plane (all structures below 104). The sacrificial layers must be removed to form the MEMS structures. This removal process is often referred to as a release process. During release process, necessary structures are often damaged because of the lack of strong protection against etchant used for release process.
When a sacrificial layer is inorganic, such as SiO2, it often provides hard, but easy to shape material and ideal for a sacrificial layer. It is also easy to remove with acid etchant, such as HF. However such etchant often leaks through a protective layer and etches and damages ILD (inter-layer dielectric) of CMOS or other types of electronic circuitry, because ILD is often SiO2 itself.
It is often difficult to find right material for a protection layer that is resistant to etchant and also electrically insulating to avoid electrical leak between electrodes. When the surface to protect is not flat and topographic, the surface is often vulnerable to the attack by etchant, even if the protective layer is resistant to etchant when it is flat. Many types of materials were tested as a protective layer, including ceramics, oxides and nitrides. These inorganic materials turned out to be vulnerable to the attack by etchant when applied with these etch stop layer formed with different topological shapes.
Therefore a need still exists in the art of applying MEMS technologies for manufacturing electronic and optical components and devices to provide a method and material to form a strong etch stop layer especially when the surface is not flat such that the above discussed difficulties and limitations may be resolved.